The isotope of aluminum having an atomic weight of 26 (Al-26) is useful in nuclear medicine and environmental chemistry. It is currently being used in research on Alzheimer's Disease and acid rain. The most readily available radioisotope of aluminum, which has an atomic weight of 28, has a half life of 2.3 minutes and is therefore not suitable for long term research. The half life of Al-26 is about 7.3.times.10.sup.5 years. Al-26 is produced at Los Alamos National Laboratory by proton spallation reactions using a silicon target. This process requires a very long irradiation time and difficult chemistry to separate Al-26 from other spallation products. This process is described in a paper entitled "Production of Microcurie Amounts of Carrier-Free Al-26", Radiochimica Acta 33, 213-215 (1983).
In the practice of the present invention, aluminum-26 is produced by spallation reactions by exposing potassium chloride to a proton beam. The proton beam used in the present invention is produced at the Meson Physics Facility of Los Alamos National Laboratory. The proton accelerator can deliver a beam of protons at an intensity of 1 milliamp and an energy of 800 MeV. Although a wide variety of studies take place in this facility, only a portion the total proton beam is depleted by the experiments. Over three fourths of the beam remains unused at the end of the experiments and continues toward the beam stop. Immediately in front of the beam stop is an isotope production facility that uses the proton beam to create radioisotopes from various target materials.
A spallation reaction occurs when an accelerated proton from the 800 MeV beam strikes the nucleus of a target atom and causes fragments of various sizes and energies to be ejected. Al-26 is one of these fragments. KCl was chosen as the target material because the larger spallation products are typically about ten to fifteen atomic units lighter from the starting materials, thus favoring production of fragments having an atomic weight of 26. Potassium has an atomic weight of 39 and that of chlorine is 34.45. The spallation process generates extremely high radiation levels and raises the temperatures of the targets to values as high as 1000.degree. C., even though they are water cooled during exposure to the beam. Irradiated targets must be handled by means of the usual methods for radioactive substances. Separation of Al-26 from the target and other fragments of the spallation reactions takes place in an isolated and shielded location called a hot cell. The work is done by means of remotely controlled mechanical manipulators behind an 18 inch thick leaded glass window.